Pin dependencies to a nixpkgs commit

shell.nix

@@ -1,6 +1,14 @@
-with import <nixpkgs> {};
-mkShell {
-  buildInputs = [
+{ pkgs ? import <nixpkgs> {} }:
+let
+  pkgsPin = import (pkgs.fetchFromGitHub {
+    owner = "NixOS";
+    repo = "nixpkgs";
+    rev = "a55ae76040581f6a69aeb2c63e4878b306c2b9ac";
+    sha256 = "0iywjwn8ksdxpfm8fan5yqdvmb91sirjgx0w3jqczza2vhxl9pf3";
+  }) {};
+in
+pkgs.mkShell {
+  buildInputs = with pkgsPin; [
     git
     go
     gotestsum